I have an equation environment where most equation lines start with an arrow, which I have aligned in the align* environment, using &. The second line in the example below is too long for the page and so I have split over two lines. Here I would like to align the start of the equations with each other rather than with the start of the arrows. I have tried using multiple & signs (and also experimented with the alignat* environment) but I cannot find a solution to this. A MWE is below.

There seems to be no distinction, in the inner align* structure, between the unary nature of the minus sign that starts the first line and the binary nature of the minus sign that starts the second line. Is that intentional?
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MicoMar 18 '13 at 15:14

@Mico ask the AMS. Semantically you're right the first one should not be a relation minus, but to me it just looks odd when you stack them on top of each other and they don't have the same spacing.
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daleifMar 18 '13 at 15:48

The following may be close to what you're looking for. Note the use of \phantom directives to create appropriate amounts of whitespace. I've created macros to typeset the subscript indices "IS" and "mix" in text-italics and text-roman, respectively, instead of math-italics, to enable text-appropriate kerning. I've also replaced \cos(\frac{\pi}{4}) with \cos(\pi/4) to make the use of larger parentheses unnecessary.

Note that the alignments aren't quite perfect across rows. This is because (i) the arrows aren't all of exactly the same length to begin with and (ii) the two minus signs in the middle equation serve as a unary operator in the first line but as a binary operator in the second (continuation) line. I'd actually recommend indenting the continuation line a bit more, to indicate more clearly its role. (Currently, the rows are aligned on the opening minus signs.)